Ukraine has arrested one Russian for spying, and expelled four others. The arrested man was identified as a colonel in the FSB (the successor to the Soviet era KGB). All five were caught near the Black Sea port of Odessa, after they had kidnapped a Ukrainian soldier, as part of an intelligence gathering mission.
Russia has greatly increased their espionage efforts in Ukraine over the last few years, in response to a more pro-West government. The Russians also resent the anti-Russian attitude of most Ukrainians (which was suppressed during the four centuries of czarist and Soviet occupation.) Russia has implied that it will invade if Ukraine joins NATO. Losing the Cold War still rankles in Russia, where memories of the mighty Soviet Union glow brighter as time passes. Regaining control of Ukraine is one of those goals, and the Ukrainians know it.